This invention relates generally to a brake-transmission interlock and particularly to a mechanism for permitting automatic transmission shifter movement from Park position only when the vehicle service brake is applied.
In automobiles with an automatic transmission, it is a preferred practice for the operator to apply the service brakes at the time the transmission selector lever is moved from Park position to one of the Drive range positions. This practice not only assures that the vehicle is held stationary during this shift, but also assures that the operator's foot does not inadvertently operate the accelerator pedal. It has been proposed to provide an interlock mechanism to prevent shifting out of Park position unless a brake is first applied.
A shift lever and parking brake control which has a pin on a shift lever movable through a shift gate having a Park position is shown in U.S. Pat. No. 4,473,141 to Mochida. An electromagnetically controlled plunger spring is biased into the gate to block pin movement into or out of the Park position. The electromagnet is electrically connected in series with a parking brake switch and an ignition switch. This causes the plunger to be withdrawn by the electromagnet to permit shifting between Park and Reverse only when the engine is running and the parking brake is on.
An electromagnetically controlled lock for a shift selector which is released for operation when ignition is turned on is disclosed in U.S. Pat. No. 3,994,183 to Markl. A brake interlock is not included in this disclosure.
In another brake-transmission interlock in current use, a pin on a shift lever movable through a shift gate having a Park position is blocked from leaving the Park position by a solenoid operated blocking member in the path of the pin. The blocking member is pivotally mounted on the same axis as the shift lever for movement between blocking and release positions. A solenoid, actuated only when the service brake is released, moves the blocking member to a blocking position and when the brake is applied, a spring moves the member to a noninterfering position.
Another brake-transmission interlock in current use is illustrated in U.S. Pat. No. 5,062,509 to Carpenter et al, and owned by the assignee hereof. In this shifter, the blocking member is pivoted commonly with the shift lever and presents a radial shoulder blocking radially inward movement of a release pin by the shift lever-mounted release button. This shoulder prevents movement of the release pin out of the Park position notch in the shift gate and subsequent movement of the shift lever out of Park position.
A problem encountered with this arrangement is that premature depression of the release pin (prior to actuation of the solenoid by depressing the brake pedal) will cause the release pin to engage the outer surface of the blocking member. This engagement produces a frictional force which can be substantial if the release button is depressed firmly. If this force is large enough, it can cause the pin to bind up the blocking member and prevent its movement out of blocking position by the solenoid.
Thus, it is desirable to provide a brake-transmission interlock having a blocking member that cannot be bound up by premature depression of the shifter release button.